Applied Passive-solar Structural Design

Passive-solar has been known to work well but certain things about our architecture fail the thermodynamic efficiency test and there are reasons why.

First is heat-transfer through conduction, the wood, drywall, sheathing all having rather high heat-transfer coefficients. Next is convection from a heat-riser which takes the warmth of the floor and moves it to the ceiling.

Until these are dealt with they cost a lot of energy, if one is trying to not require external heating for a building.

A first principle to recognize is that we need to insulate the OUTSIDE of a building to prevent heat-transfer from anything nailed to the studs which act like pathways for heat to be lost to "radiators", the sheathing & siding of the building. Insulation between the studs doesn't do much to prevent this, a direct insulation of exterior heat-transfer surfaces needs to be added, using insulated sheathing is the simple way to do this.

The second is if you don't collect heat you can't create an autonomous building that maintains comfort zone, followed by if you don't have enough thermal mass to heat during the day the building will cool back down before dawn.

Those are the three pieces to use in any situation but standard framing and insulation techniques need to be altered to gain this idea in a design.

The largest single key for comfort is to create and circulate warm air during the day that heats up the thermal mass of the building. If you use insulation on the outside of the structure all of its mass becomes thermal-mass to maintain comfort zone for the interior instead of being able to conduct heat to the outside world.

Then, a greenhouse wall is mandatory in passive solar, so how to get it to function as an air heater requires two panels with an air gap to warm the air well, this is needed to force a flow from floor to ceiling, but that's not good enough, the warm air from the ceiling needs to be drawn all the way down to not lose too much by it all gathering high in the room.

This implies using a slightly different way of building. First the typical plywood sheathing is used on the INSIDE of the walls opposite where the greenhouse wall is with joists between the roof-bearing wall used as ductwork so between studs is open near the ceiling with inlet vents and inside the wall open to the floor joists inside the walls & floor so the air must pass through the floor joists to get to the double greenhouse panels that heat it. If done correctly this air-flow will not require fans. The floor joists are covered by exterior insulated sheathing to open the space to this airflow.

The result is a space heater built into the home that keeps the floor warm on a daily basis, uses all the mass of the building as thermal storage by insulating outside the structure and avoiding easy heat-transfer paths. I'm working on ceiling systems that fit into this scheme, to gather heat or cold for the needs of the season but wanted to put these concepts out to people on the list to consider in their remodels & building from scratch.

Replies to This Discussion

Tom - I get it; you will be capturing more heat from solar than you will be losing. But at what cost? If you total all your efforts (assign an appropriate $ value to all your planning/labor - including any on-going labor to keep your solar project running properly, total all your purchases/contracted effort) versus how much cost you have avoided (your previous space heating costs) I believe you will have far exceeded any respectable ROI. This assumes that your system works as planned. And it will not help you with Summer conditioned air needs. You may be better off investing the same funds in a money market account.

However if you invest a fraction of that time/energy/funds into keeping the conditioned air/temperature in the residence, your ROI would be about 1-2 years, have no on-going maintenance costs and be better than any financial institution on the planet. And your comfort level will improve whereas your proposal will still leave you with an uncomfortable house because the basic problem issues will not be adequately addressed. Check out the references. Check out the Krigger/Dorsi book that is the definitive source taught at almost every home performance certification class.

Back to first base David, this is a 12x12 cabin that I'm building not a hypothetical situation. You constantly bark about ROI so here's how you're off a long way on that and now I want you to put up some numbers instead of words.

1. The greenhouse wall is really there just to have a view, I'm using it to heat the cabin, the main goal is the view, not the passive-solar, the windows are plain-glass for heat production by incident angles not normal to the surface but that also give the best view, lucky on that.

2. I'm adding the expense of a second glazing separate from the thermo-panes to get a way to power air circulation, this is innovative, nobody uses the strategy so this is an experiment in that respect. To keep costs down there is no change in framing lumber, none, minor assembly details are different but costs are identical yet the framing technique gains complete ductwork for the air circulation.

3. Since we're talking ROI the plywood sheathing is on the inside, no drywall, so subtract the cost of drywall and the cost of 3-1/2" batting that used to go in the wall countered by extra cash to buy insulated sheathing on the outside.

4. The side walls get R19, the roof, floor and north wall all have R30 in foam so that costs more than the batting or attic treatment found in most existing homes but this will be less expensive than your treatments.

5. I'm adding plastic pipes filled with water for thermal-mass, this will cost more than standard construction.

6. Needed are temperature activated vent-closer pistons to take advantage of a hot roof and add gain from that into the internal circulation, and to close off things when it cools down so the circulation doesn't reverse, costs more, pistons are $25 each.

To sum up I've used foam instead of batting and that costs me money, an extra 96-sf of single-pane glass to get a collector, a bunch of pvc pipes for thermal-mass, and two thermal actuated pistons for automatic circulation control.

That's the extra I've spent up front for this, all else will be the same as a standard building with batt insulation, 2x4 walls, 2x10 joists floor & roof.

So, my challenge to you David is to show how you're going to come up with the 5,760-watts a day with the expenses you list adding-subtracting to what I'm doing that bring this up to your standard which you claim is a great return on the dollars spent, the ROI.

Keep in mind that there is no heater, no utilities, it's a mountain ridge, only a steep trail up to it and everything has to carried up on our backs. Seems you forget gaining heat in design work to me and think just insulating is great because your bills go down.

I disagree with you David, I think your approach is simplistic to the overall goal of autonomous buildings.

I'm assuming all previous building descriptions apply to this 12x12 cabin (i.e., not new construction). I'd propose: sealing the crawlspace dirt floor, sealing rim joist area and insulating the crawlspace walls as previously described (keeps ground moisture in the ground; does not expose it to ambient air for mold/fungus growth; moves the conditioned space to include the crawl) - you can use the space for whatever afterward; remove all North/East/West windows and convert the area to wall (too hard to regulate E/W solar gain; N exposure is a total loss); fill all interior cracks with silicone caulk or expanding foam (see referenced sources for air barrier checklists); R-19 sidewalls and R-30 ceiling will not even pass code for new construction so it is not better - increase side wall R value by applying 2 layers of R-12 polyisocyanurate foam sheeting (tape all joints with UL-181, stagger all joints), apply water drainage plane material, fasten 1x3 strapping with epoxy-coated screws for a siding base (see Mass. barn retrofit @ Building Science Corp.), seal off any soffit areas (plain construction/vents/etc) and apply foam panels to roof deck to achieve at least R-49 (more is better), apply water drainage material/strapping/roofing material (similar to walls; check references for examples); install un-faced fiberglass insulation in roof joist areas; apply (glue/screw) OSB to underside of roof joists, apply duct mastic or tape to joints to create new air seal (not the ceiling below) tied to the interior walls - use attic area to run wiring/venting/etc as this is now conditioned space; instead of plywood on the interior walls, use OSB (much better air seal) gluing it to all wall studs (constructs a "torque box" for stability), tape all seams (or use duct mastic) to create air barrier tied to roof air barrier; run surface wiring (or create additional hollow wall for wiring) so as not to penetrate the air barrier; anything that penetrates the air barrier (vent stack, etc.) must be sealed at the air barrier layer; integrate an air-to-air energy recovery ventilator unit (maybe solar-charged batteries if you are off the grid) with ducting to all rooms and pickup points at kitchen and bath (no kitchen/bath fans); DO NOT add any combustion appliance (stove, heater, range, water heater, etc.) that does not exhaust directly to the outside and receive combustion air from the outside; run any plumbing in interior walls; install insulated, tight-fitting window treatments on remaining glazing.

I'm guessing that you will not need any additional solar heat for space heating (but maybe for DHW) so you can eliminate all the other expenses for air circulation/storage/regulation/etc.; even the second layer of glazing.

This is not simplistic; it is modern building science (much of which has only really come to light the past 10 years or so). Look at the research material. Pay attention to details. Design for minimal air/thermal loss and proper (not random) air exchange.

David, what does that typically cost a square foot at today's pricing? I want your best ballpark low-to-high of getting it done by a contractor or DYI'er.

It's a much bigger list than I have for sure, but, I see nothing on solar-gain or thermal-mass in the list so even with this dandy upgrade and super-sealing it's a heat-loser!!

So you're bills did go down heavily but you're not off-the-grid to stay warm so I fail to see where what you descibe fits a ridge-top cabin with no utilities that'll have a wood stove?

To get more technical about how much insulation you need for any home an issue with me is that you not have studs with outside sheathing on them, this forces the heat-transfer into a much smaller area on the exterior facing walls, so every 16" you have a stud of heat-transfer trying to get through the outside insulation instead of transferring heat from the studs to sheathing on the outside which acts like a radiator so you radiate from the studs to 100% of the wall exterior, insulated or not.

What that means is my R19 is worth R30 on a side wall by thermal design, about a third more efficient by how the pieces are placed. Is there a coefficient you use for assorted wall gemetries or do you analyze those? It makes a huge difference in how much insulation you need by disallowing heat-flow paths.

But, the design still loses a ton through the windows and that's why thermal-mass is important to make a daily cycle and why I'm adding more water-filled pipes [now up to 1,200kg of water storage].

And, you keep wanting to cover the crawl space but your designs do not have thermal-mass storing heat in the floor radiating 24x7 to keep it warm.

That's a key issue with me now, by having it there the crawl-space soil becomes a thermal-mass that helps maintain the heat of the home over time by moving the equalibrium point far enough away to operate in this fashion. Just like a dug-in home but it's below the floor where you want it.

As I stated, what I suggest doing is dig up a couple feet of it and place insulation down with the dirt back on top of it to contain it specific as storage for the home above instead of continuing to heat soil over time.

This for homes is cheap thermal mass to consider gaining if you're installing heat distribution and thermal-mass below the floor, otherwise trying to seal it all off works fine and far better than doing nothing as many older homes show.

Tom, I've been wanting to add a passive solar section to my educational energy site, Johnny on Energy, and I'm looking for opportunities to film videos of homes or additions that are complete or under construction. I also have a personal interest as I intend to downsize and build a passive home when my two college studuents graduate. This PV solar home tour is a good example of a video on my site Solar Residential in which a company called Alt Energy provides the tour. I'm located in Central, VA. So there is no confusion, we do not charge anyone to be on our site. We're happy to have them contribute and we always put in a good word for them and link back to their sites so that the reader can learn more.

If you're reading this and you're an energy auditor in my area I'd also like to add a few videos on the value of a home inspection and what the home owner can expect. There is a contact link on my site.

As a design problem I considered the typical Arctic flown-in cabin on piers during winter, no sun. Working on improving that led to knowing that the only rational place for thermal mass is under the floor, so, this is an attempt at actively moving the heat back down below the floor at mid-latitude all year, homes need a thermal bank for inertia to a change in temperature.

This page has been edited since making it, but close on figuring out circulation from thermal gain with volumes, used Bud's diagram to base pressure. The solar-gain wall produces about 64-cubic feet of flow per hour, expanding 2-1/8" over 8ft in the 1/2" space gaining 100F and at 8ft venting into the room of 1,232-cf with a peak at 12ft connected when conditions allow to the roof-space above the insulation of 55.2-cf, return inlet to the opposite wall at 10ft having 27.2-cf of thermal-mass, its air-volume 6.6-cf, this the vertical wall that spans the change in temp from ceiling to floor, then back under the floor of 78-cf air 104-cf thermal-mass to the inlet of the solar wall. Air is never below 40% humidity and above 60% most of the time.

Detail drawing of a construction method to direct flow to under the roof when it's hot into the room circulation using a custom vent with a greenhouse plunger to open-close with temp changes. For a cabin on a forested ridge the roof offers a lot of heat when the sun is overhead for a couple hours with no shade, too big an opportunity to gain heat to ignore with a shaded site.

Posted all the drawings to Picasa, the figures are all over and end up being the lumber list, a pressure diagram, heat-transfer and building details to have the whole work as a circulation.

By adding another 1-1/2" insulated board 1/2" below the roof plywood when the sun is overhead it adds to the gain, using greenhouse plungers to open/close a custom vent for this.

The pipes between joists & studs hold about 4-tons of water as thermal mass, thermo-panes with standard glass on the window wall with an added single layer to create the air-gap for heating the first 8-ft, manual insulation to close when no one is there for the top 4-ft sections.

Hi Tom, this is a great idea that has been in use for at least 100 years, but is rarely used because it is poorly understood, so I love that you guys have dissected and explained the science behind it. The nea-sayers have not tried it.

Last winter I installed something similar on an off-grid clients home and they are over the moon. They only have wood heat, so they really appreciate not building fires in the shoulder seasons, but the performance in the heart of winter is only marginal because of glazing losses.

The other great solar collector is your roof. A small fan with a differential temp controller blows warm air from the attic into the house. Also great for shoulder seasons, but not so good when it is -20F.

Thank you Bill, using the air-space between studs is more like Balloon Framing for sure. We'll see what the spiders do ... the floor insulation is removable for cleaning or if a pipe leaks.

Using pvc pipes for thermal-storage is the feature that matters most tho', collecting heat is pretty easy. For control, the roof vent shuts down at 40F, yet on a clear day in winter the solar wall gains ... we'll see how it goes.

I think for a home where the crawl-space was being revamped it makes sense to install the pipes and use forced-air to move warm air to store heat below the floor.

A warm floor is so rare, it's amazing the difference in the room because cold air can't sit down low.

I also think the roof collection will tip the scale but we'll see. Latest thoughts are on how to finish the exterior to look forest-like.

Hi Tom, I looked over your plans a little closer this morning and there are a couple other comments that I would like to make.

PVC is not a good choice for water pipes, since it has a low heat transfer coefficient, is prone to fracture spirally if the water inside freezes and they are toxic. Black poly-pipe is fine for an open loop system (if you are going to use on open unpressurized tank or fountain), but if you are going closed loop, then you want PEX.

Solar gain on clear days will make the cabin oh so warm and toasty, but what about the weeks where the wind blows 70 mph and the temps stay below zero? Then some of Dave's insulation and air sealing details become important.

I love clay! I live in a clay house and I use clay on almost every remodel I do. Clay is THE BEST thermal storage medium, with the added benefits of low cost, low maintenance and RH moderation. I guess what I'm trying to suggest is less water and a clay floor and/or walls.

For your exterior, I would recommend local pine or fir installed vertically over horizontal battens. I like 1x10 boards caulked together and 1x3 battens over the seams. Then stained with Penofin Brazilian Rosewood oil which blocks 99% UV.

Good luck, it's great to see somebody really thinking outside of the box.